Recently, the use of supercritical fluids such as supercritical carbon dioxide, etc. as solvents has been vigorously studied. While supercritical fluids have a surface tension of zero and thus can be as well diffused as gases, such fluids can be used as solvents because of their densities close to those of liquids. As one of the novel production processes by making effective use of the physical properties of such supercritical fluids, there is proposed nonelectrolytic plating of plastic molded articles (cf. Non-Patent Publication 1). Nonelectrolytic plating with use of a supercritical fluid makes it possible to overcome the following problems of the conventional technology of the nonelectrolytic plating of plastic molded articles.
The conventional nonelectrolytic plating is widely employed as means for forming metal films on resin molded articles for electronic equipment, etc. In general, a conventional nonelectrolytic plating process comprises the steps of molding a resin, degreasing the resin molded article, etching the molded article, neutralizing and wetting the etched molded article, adding a catalyst, activating the catalyst, and subjecting the molded article to nonelectrolytic plating, while there may be some difference in the steps, depending on materials to be used.
In the etching step, a chromic acid solution, an alkali metal hydroxide solution or the like is used.
Therefore, a post-treatment such as neutralization of an etchant is needed in the conventional nonelectrolytic plating, and such a post-treatment becomes one of factors of high cost. The use of a highly toxic etchant in the etching step induces problems in handling of the etchant. In the Europe, there was constituted the regulation of RoHS (RoHS: Restriction of the use of certain Hazardous Substances in electrical and electronic equipment). Under this regulation, the manufactures of the materials and the electric and electronic components have been obligated to guarantee that electric and electronic equipment newly put on the European market after Jul. 1, 2006 should contain no chromium (VI) or the like. It is also an urgent mission for the manufactures to change the conventional nonelectrolytic plating of plastics which heavily burdens the environment, over to an alternative nonelectrolytic plating process.
According to the process disclosed in Non-Patent Publication 1, an organic metal complex is dissolved in supercritical carbon dioxide, and a variety of polymer molded articles are brought into contact with this supercritical carbon dioxide. By doing so, the organic metal complex is infiltrated in the surfaces of the polymer molded articles. The polymer molded articles infiltrated by the organic metal complex are further treated by heating or chemical reduction, so that the organic metal complex is reduced to deposit fine metal particles. A sequence of treatments as described above modify the surfaces of the polymer molded articles so as to enable nonelectrolytic plating on the polymer molded articles. Since this process comprises no etching step, any treatment of the waste of the etchant is not needed, differently from the conventional nonelectrolytic plating. It is also not needed to roughen the surfaces of the molded articles with the etchant so as to ensure tight adhesion of the plated films to the molded articles. Therefore, the surfaces of the molded articles and the plated films are superior in smoothness to those obtained by the conventional nonelectrolytic plating with the use of an echant.
However, the nonelectrolytic plating with the use of the supercritical fluid, disclosed in Non-Patent Publication 1, has the following problem: the polymer molded article is softened at its surface by the supercritical carbon dioxide after the molding step, to thereby infiltrate the supercritcal fluid and the metal complex as a modifier in the polymer molded article. Consequently, the contour of the molded article deforms due to such softening, and thus, molding precision of the molded article can not be maintained. The nonelectrolytic plating with the use of the supercritical fluid, according to Non-Patent Publication 1, is poor in continuous productivity, because this plating is a batch process in which polymer molded articles are set in a high-pressure container so that the metal complex is infiltrated in the polymer molded articles. This plating is also unsuitable for plating of large-size molded articles, since a high-pressure container corresponding to such large size is needed.
The present inventors have proposed a method for modifying the surface of a molded article so as to enable nonelectrolytic plating on the molded article by applying this process principle to segregate fine metal particles on a plastic molded article in injection molding (Patent Publication 1), This is described in detail: for example, fine metal particles of a metal complex or the like are dissolved in a high-pressure supercritical carbon dioxide; this solution of the supercritical carbon dioxide is charged in an injection molding apparatus so as to introduce the supercritical carbon dioxide into the flow front portion of the thermoplasticizing cylinder of the injection molding apparatus; and this thermoplastic resin is injection-molded, so that the fine metal particles are segregated on the molded article simultaneously with the injection molding. Thus, the fine metal particles which act as catalytic nuclei for nonelectrolytic plating can infiltrate the molded article concurrently with the molding, and additionally, the fine metal particles can be segregated on the surface portion of the molded article. Moreover, pre-treatments for plating such as the above-described steps for infiltration and etching are not required between the molding step and the plating step.                Non-Patent Publication 1: “Latest Applied Technology of Supercritical Fluid” by Teruo Hori, issued by NTS Co., Ltd., pp. 250 to 255, 2004        Patent Publication 1: JP-B2-2625576        